Field of the Invention
The invention relates to an integrated semiconductor circuit having a semiconductor substrate; at least one first potential rail which carries a first supply potential of the semiconductor circuit in operation; at least one second potential rail which carries a second supply potential of the semiconductor circuit in operation; at least one first circuit portion for receiving and processing input signals; at least one second circuit portion in which at least one output signal of the semiconductor circuit is developed in operation of the semiconductor circuit; and a configuration for protection against overvoltages, which includes a first protection circuit for each terminal for a line carrying one of the input signals, which is disposed between the respective terminal and a respective first circuit portion and which includes a field oxide transistor and a field-controlled diode disposed electrically parallel with one another between the line carrying the input signal and the first potential rail, the gate of the field oxide transistor is connected to the line, the field control of the diode is effected through the first potential rail, and the field oxide transistor and the field-controlled diode are electrically connected to the line carrying the input signal at various points of that line; the line carrying the input signal includes a diffusion resistor between the two points; a respective tub-shaped region is formed in the field oxide transistor, beneath the source region and beneath the drain region, which regions are spaced apart from one another and are both of the same conduction type as the source and drain regions.
One such semiconductor circuit is known from U.S. Pat. No. 4,987,465 and German Published, Non-Prosecuted Application DE 39 18 090 A1. Integrated semiconductor circuits, in particular MOS integrated semiconductor circuits, are known to be vulnerable to electrostatic discharge (ESD), if such discharges act on their terminals (pads on the semiconductor chip, and pins on the semiconductor circuit housing). Tests of known semiconductor circuits have shown that although they have improved protection against ESD effects as compared with the past, nevertheless the protection is still not sufficient to meet current wishes.